Seasonal Forecast

Regular readers will know that I have strong personal and professional interests in long-range forecasting, including both sub-seasonal (weeks ahead) and seasonal (months ahead) outlooks.  It's a challenging discipline, of course, but seasonal forecasting in particular is a mature field with well-established methods and tools that (can) lead to a modest degree of success.

Among the tools that are regularly consulted by seasonal forecasters are both dynamical model forecasts produced by well-funded supercomputing centers and statistical forecasts from any and all sources.  An attractive aspect of statistical techniques is that they can be simple enough to run on a desktop computer in minutes; there's no barrier to entry in this field.

In the past week I've been digging into a new approach reported by Matti Kämäräinen of the Finnish Meteorological Institute:

https://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-18-0765.1

I won't reproduce details, but the essence of the scheme is that it develops a large ensemble of regression models over a long 70-year history to predict surface temperature in 3-month periods.  The predictors are simple: global sea surface temperature and 150mb (lower stratosphere) geopotential height patterns in previous 3-month periods.

Kämäräinen and his co-authors report significant skill in predicting area-average temperature over large domains such as Scandinavia and western Europe.  I managed to reproduce most of the results with my own code and then apply the scheme to a domain over Alaska and western Canada.  The model produces forecasts for the area-average temperature in terms of departure from trend, and the chart below shows results for the entire verification period, 1986-present (click to enlarge).

Here's the forecast domain: note that we're predicting area-average temperature for the whole region, not for any particular point.

The overall correlation is modest but unquestionably positive.  Here are the correlation coefficients for the last 20 years of 3-month forecasts, i.e. 80 consecutive seasonal forecasts for 2000-2019:

Dec-Feb  +0.66
March-May  +0.35
June-Aug   +0.58
Sep-Nov   +0.36

Bearing in mind that this is strictly out-of-sample verification (i.e. we're not cheating), and that the long-term trend was removed (i.e. the trend does not boost the skill), this is quite impressive in my view.

So what does the scheme predict for this summer?  The last data point on the chart shows the forecast for June-August 2020, and it's a cool look; in fact, the regression predicts the lowest trend-relative temperature for summer since 2009.  The cool forecast doesn't look like much on the chart, because variance is low in summer, but it's a big change from last year's forecast, which was (correctly) quite warm.

Given that the latest dynamical model forecasts are pointing warm, and NOAA's Climate Prediction Center also shows warm, it will be interesting to see how the summer turns out.  Of course, a true comparison of these forecasts would require us to reconcile the different climate reference points, because "below trend" could easily be "above normal" if we're talking about the 1981-2010 baseline that CPC currently uses.  I'll leave that detail for another day.

Tripod Out

The Nenana tripod went out yesterday, marking the break-up of the Tanana River at its confluence with the Nenana River.  As I noted last week, it's highly likely that the break-up was earlier than it would have been without such abundant run-off from excessive late winter snowfall across the region.

This claim is supported by the fact that this year's break-up occurred with the least number of thawing degree days (TDDs) on record in nearby Fairbanks: the total TDDs through yesterday in Fairbanks was 74.0, which narrowly beats out 2002 (74.5) and 2006 (75.0).  We might say this year's thaw season leading up to break-up was the coolest on record.

Here's an updated chart of the relationship between TDDs and precipitation amount from March 1 to break-up.

An interesting nuance here is that most of the years with high precipitation and low TDDs also saw relatively late break-up, in contrast to this year.  The chart below classifies the years as "early" or "late" simply based on whether break-up was before or after the long-term median of May 3 (and yes, there's a significant trend).

It seems that it's typical for wet years to also be cool in April, and this delays break-up in comparison to dry years; i.e. dry and warm tend to occur together in late winter and early spring.  This complicates the picture a bit, because the stronger sunshine at later dates allows late break-ups to occur with lower TDDs independently of precipitation effects.  So the precipitation/TDD relationship isn't quite as simple as "more run-off means earlier break-up"; the relationships are complex.

A final bit of chart analysis (see below) illustrates that the very latest break-ups reliably occur after low TDD totals; but this year was unusually early for such low TDDs.  I suggest this is almost certainly because of the increased run-off related to the wet conditions of the past 6 weeks.

Length of Summer - Brian B Blog

Most readers will be familiar with Dr Brian Brettschneider of UAF, who has become a sought-after authority on Alaska climate.  Brian recently wrote an article on his own blog that I thought would be worth linking here:

Are Summer/Winter Longer/Shorter Than They Used To Be?

Using a simple method, Brian demonstrates that the "summer" and "winter" temperatures of a prior climate period (1960-1989) in Alaska have become considerably longer and shorter respectively in the most recent 30-year period (1990-2019).  The changes have been particularly dramatic in winter across Alaska, and most notably of course on the North Slope.

Brian's work provides a nice visualization and easy-to-understand interpretation of temperature changes over the past several decades.  In my opinion, it would also be interesting to extend the work to build in the different perceived length of summer/winter at different latitudes.  For example, the introduction acknowledges that Houston has a longer summer than Omaha, and Chicago has a longer winter than Oklahoma.  Most people would say Alaska has a very long winter and a short summer; and despite the dramatic warming on the North Slope, winter certainly hasn't disappeared there, as Brian's simple method would imply.

Obviously it would be difficult to come up with a good metric to capture subjective opinions of summer and winter, but perhaps a threshold temperature could be added that would demand a "summer" or "winter" classification, regardless of how long that season might be.  For example, perhaps:

- Normal daily mean temperature above 27°C -> summer
- Normal daily mean temperature below -10°C -> winter

The winter threshold would then produce a (1981-2010 climate) winter season from October 28 to March 21 in Fairbanks, and October 22 through May 3 in Utqiaġvik.  Perhaps readers would like to comment on this or other ideas to refine the analysis without losing too much of the attractive simplicity.

Lots of Water

Interior Alaska is finally escaping the grip of winter, with snow disappearing quickly and break-up beginning to advance on some rivers.  Remarkably abundant snowfall in late winter combined with recent rains has led to an excess of run-off and the potential for flooding if temperatures rise too high too quickly.  Here's the NWS flood potential map from last week:

Fairbanks is already down to a snow depth of only 4 inches, but the snow pack remaining in the hills is quite monumental: the Munson Ridge SNOTEL site at 3100' elevation near Fairbanks is reporting 15" of liquid equivalent water.  That's a lot of water to come down the creeks and rivers in the next month.

According to the SNOTEL instrument, 6.8" of liquid-equivalent precipitation has fallen on the hill since March 1 - this is quite remarkable for the (usually dry) time of year.  It's believable too; here's a chart of recent precipitation at the top-quality CRN site (1140' elevation).

The massive influx of melt water will almost certainly bring forward the date of break-up at the Nenana tripod, owing to the increased stress caused by high water.  There's some evidence of this effect in the history of the Nenana break-up; the chart below shows that when precipitation is very high after March 1, the heat input (thawing degree days) required to reach break-up tends to be lower than normal.

This year Fairbanks has seen 2.86" of precipitation since March 1, which is the highest on record for March 1 - April 21.  The previous record for this period was in 1967, which happens to be the year of the great August flood in Fairbanks (hmm...)

Here's the latest break-up summary from the NWS.

HYDROLOGIC OUTLOOK
NWS ALASKA PACIFIC RIVER FORECAST CENTER
ANCHORAGE AK
1 PM AKDT WED APR 22 2020

..SPRING BREAKUP OUTLOOK FOR ALASKA...

Breakup Update: The Forty Mile river upstream of the Taylor Highway
has broken up as well as the Kuskokwim River near Nikolai. The Tozitna
River has broken up with an ice jam forming and releasing. The Aniak
River is reported to have broken up dynamicaly and is mostly open at
the confluence with the Kuskokwim. Ice is beginning to lift on the
Kuskokwim, Yukon and Koyukuk Rivers.

The 2020 Alaska Spring Breakup flood potential is forecast to be
generally above average south of the Brooks Range and average flood
potential for the North Slope. The Flood Potential forecast is based on
observed snowpack, ice thickness reports, and long-range temperature
forecasts and focuses on the Kuskokwim, Tanana , Yukon, Koyukuk,
Copper, and North Slope rivers. Recent observed and forecast
temperatures suggest a generally earlier than normal, more dynamic
type breakup for locations statewide.

Timing of breakup statewide is expected to be several days earlier
than normal. Breakup along the Yukon River upstream of Fairbanks
is expected to be 1 to 3 days earlier than median dates; generally 3 days
earlier at locations downstream from Fairbanks to Anvik; and 3 to 5 days
earlier at locations downstream from Anvik to the mouth. Breakup along the
Kuskokwim River at Nikolia downstream to Tuluksak are expected to be 1
to 2 days earlier than median dates and 2 to 5 days earlier downstream
from Tuluksak to the mouth.

Temperatures - Statewide, temperatures over the past few weeks generally
have been normal to below normal. However, temperatures over the next
several weeks are expected to be normal in areas north of the Brooks
Range and above normal for the remaining parts of the state. Statewide,
temperatures for April and May are expected to be above normal.

Ice - The April 1st ice thickness data indicate that ice thickness is near
normal across the state. A few measurements across interior Alaska
ranged from 32 inches at Galena, which is 82% of average; 33 inches
at Nenana, which is 80% of average; 48 inches at Eagle, which is
117% of average.

Snow - April 1st snowpack by the Natural Resources Conservation Service
(NRCS) indicates greater than normal snowpack across interior Alaska
ranging from 117% in the central Yukon Basin to 197% of average in
the Kuskokwim Basin. The Yukon government is also reporting greater
than normal snowpack across the Upper Yukon River Basin in Canada.
The snowpack is near normal along the North Slope and below normal along
the Gulf Coast and throughout Southeast Alaska.

Climate Outlook:

April and May weather is the most important factor determining the severity
of river ice breakup. Dynamic breakups have a high potential for ice jam
flooding and typically require cooler than average temperatures during March
and into the first few weeks of April followed by an abrupt transition to
warmer than normal temperatures in late April to early May. Thermal breakups
have a low potential for ice jam flooding as river ice generally rots in place.

The Climate Outlook for Spring 2020 suggests a more dynamic type breakup this
year. Observed temperatures in March thru the first week of April were generally
below normal to normal. The second and third weeks of April were generally
warmer than normal.  Temperature forecasts for the last week in April
indicate near normal temperatures statewide with equal chances for above and
below normal for early may. The mid-March 3-month Climate outlook
indicates increased chances of above average temperatures throughout Alaska.

Further information can be found at the following web sites:
Weather-
https://www.weather.gov/aprfc/AGAK78PACR

Snow-
https://www.weather.gov/aprfc/nrcs_ak_swe
https://www.weather.gov/aprfc/snow_depth
https://www.wcc.nrcs.usda.gov/snow/

Ice thickness-
https://www.weather.gov/aprfc/icethickness

Climate prediction-
http://www.cpc.ncep.noaa.gov/

River Ice Breakup Flood Threat:

The two generalized types of river ice breakup are dynamic (mechanical)
and thermal. A dynamic breakup moves from the headwaters of a river
downstream in a somewhat linear fashion. Ice jam flooding occurs more often
during a dynamic type breakup than a thermal type. A thermal type breakup
results in the river ice rotting in place primarily due to gradual to rapid
warm ups and little snowmelt runoff. Thermal type breakups usually result
in fewer ice jams and less chance of flooding.

Statewide, breakup this year is expected to lean generally to the dynamic
type. Temperatures statewide over the past few weeks generally have been
normal to below normal. However, temperatures over the next several weeks
are expected to be normal in areas north of the Brooks Range with above
normal temperatures for the remaining parts of the state. Temperatures
statewide for April and May are expected to be above normal. South of
the Brooks Range, the expected more dynamic type breakup, above normal
temperatures, and above average snowpack has increased the potential for
flooding this year to above average, while flooding potential north of the
Brooks Range is expected to be average.

THE FOLLOWING TABLE GIVES AN ESTIMATION OF FLOOD POTENTIAL FOR
VARIOUS LOCATIONS AROUND THE STATE AND BASIN RUNOFF VOLUMES.
---------------------------------------------------------------------
SNOWMELT RUNOFF VOLUME...EXPECTED WATER VOLUME FROM SNOWMELT DURING
                         THE MELT SEASON.

FLOOD POTENTIAL...THE LIKELIHOOD OF FLOODING FROM SNOWMELT AND/OR
                  ICE JAMS.  THIS IS INITIALLY CALCULATED BASED ON THE FLOOD
                  FREQUENCY FOR THE 2000 TO 2019 HISTORICAL RECORD AND ADJUSTED
                  TO REFLECT CURRENT CONDITIONS.

*  MEDIAN BREAKUP DATES ARE FOR THE PERIOD 1980 THROUGH 2019 AND ARE
      CALCULATED FOR LOCATIONS WITH AT LEAST 5 YEARS OF DATA.
** ACTUAL BREAKUP DATE FOR THIS YEAR
---------------------------------------------------------------------

                      SNOWMELT    FLOOD     MEDIAN  NO. OF  FORECAST
RIVER - REACH          RUNOFF    POTENTIAL  BREAKUP  YEARS   BREAKUP
                       VOLUME                DATE*    USED    DATE
-------------         ---------- ---------  -------  ------  --------
SOUTHEAST PANHANDLE     ABOVE

KENAI RIVER            AVERAGE    LOW       OPEN TO SKILAK LAKE

ANCHOR RIVER            BELOW     LOW       MOSTLY OPEN

MATANUSKA RIVER       ABOVE AVERAGE  LOW

SUSITNA RIVER         ABOVE AVERAGE
  GOLD CREEK                      MOD
  SUNSHINE                      LOW-MOD     05/02     31  04/26-05/02

YENTNA RIVER          ABOVE AVERAGE
  LAKE CREEK                    LOW-MOD     04/30     29  04/27-05/03

SKWENTNA RIVER        ABOVE AVERAGE
  SKWENTNA                      LOW-MOD     04/29     25  04/23-04/29

COPPER RIVER BASIN     AVERAGE
  GAKONA RVR @ HWY                LOW       04/29     32  04/23-04/29
  GULKANA RVR @ HWY               LOW       04/29     30  04/23-04/29

CHENA RIVER           ABOVE AVERAGE
  CHENA LAKES PROJECT             MOD
  FAIRBANKS                       LOW       04/25     27    OPEN

TANANA RIVER          ABOVE AVERAGE
  CHISANA @ NORTHWAY             LOW-MOD    04/26     27  04/17-04/23
  SALCHA                         LOW-MOD
  FAIRBANKS                      LOW-MOD    04/28     19  04/26-04/02
  NENANA                         LOW-MOD    04/29     40  04/24-04/30
  MANLEY                         LOW-MOD    05/02     29  04/26-05/02

KUSKOKWIM RIVER       ABOVE AVERAGE
  NIKOLAI                        LOW-MOD    04/22     34   **04/21
  MCGRATH                          MOD      05/04     40  04/30-05/06
  STONY RIVER                    LOW-MOD    05/01     32  04/27-05/03
  SLEETMUTE                        MOD      05/01     31  04/27-05/03
  RED DEVIL                        MOD      05/03     34  04/28-05/04
  CROOKED CREEK                    MOD      05/03     34  04/29-05/05
  ANIAK                            MOD      05/04     37  04/29-05/05
  KALSKAG                        LOW-MOD    05/04     31  04/30-05/06
  TULUKSAK                       LOW-MOD    05/06     28  05/01-05/07
  AKIAK                          LOW-MOD    05/08     34  05/01-05/07
  KWETHLUK                         MOD      05/03      8  04/30-05/06
  BETHEL                         LOW-MOD    05/09     40  05/01-05/07
  NAPAKIAK                       LOW-MOD    05/10     25  05/03-05/09

YUKON RIVER (UPPER)   ABOVE AVERAGE
  DAWSON, YT                     LOW-MOD    05/03     40  04/29-05/05
  EAGLE                            MOD      05/03     40  04/28-05/04
  CIRCLE                         MOD-HIGH   05/07     36  05/02-05/08
  FORT YUKON                       MOD      05/10     36  05/04-05/10
  BEAVER                         LOW-MOD    05/09     24  05/04-05/10
  STEVENS VILLAGE                LOW-MOD    05/10     23  05/05-05/11
  RAMPART                        LOW-MOD    05/10     25  05/04-05/10

YUKON RIVER (MIDDLE)   ABOVE AVERAGE
  TANANA                         LOW-MOD    05/07     35  05/01-05/07
  RUBY                           LOW-MOD    05/08     34  05/02-05/08
  GALENA                           MOD      05/10     39  05/04-05/10
  KOYUKUK                          MOD      05/08     14  05/04-05/10
  NULATO                           MOD      05/10     23  05/04-05/10
  KALTAG                         LOW-MOD    05/11     34  05/05-05/11
  ANVIK                          LOW-MOD    05/13     32  05/07-05/13

YUKON RIVER (LOWER)    ABOVE AVERAGE
  HOLY CROSS                     LOW-MOD    05/14     33  05/06-05/12
  RUSSIAN MISSION                LOW-MOD    05/14     34  05/06-05/12
  MARSHALL                         MOD      05/14     28  05/06-05/12
  PILOT STATION                  LOW-MOD    05/13     23  05/07-05/13
  MOUNTAIN VILLAGE               LOW-MOD    05/15     33  05/08-05/14
  ALAKANUK/EMMONAK                 MOD      05/20     32  05/12-05-18

KOYUKUK RIVER          ABOVE AVERAGE
  BETTLES                        LOW-MOD    05/09     38  05/01-05/07
  ALLAKAKET                      LOW-MOD    05/09     33  05/03-05/09
  HUGHES                           MOD      05/10     33  05/03-05/09

SEWARD PENINSULA       ABOVE AVERAGE
  BUCKLAND                         MOD      05/15     30  05/09-05/15

KOBUK RIVER            ABOVE AVERAGE
  KOBUK                            MOD      05/13     36  05/07-05/13
  SHUNGNAK                       LOW-MOD    05/16     29  05/10-05/16
  AMBLER                         LOW-MOD    05/16     35  05/10-05/16

NOATAK RIVER           ABOVE AVERAGE
  NOATAK                         LOW-MOD    05/13     23  05/17-05/19

BROOKS RANGE - NORTH     AVERAGE
  COLVILLE @ UMIAT               LOW-MOD    05/24     19  05/16-05/22
  COLVILLE @ COLVILLE            LOW-MOD    06/02     22  06/26-06/01

SAGAVANIRKTOK RIVER      AVERAGE
  DALTON HWY                     LOW-MOD


For more detail and to see a Flood Potential Map, refer to our Website
at https://www.weather.gov/aprfc/FLOODPOTENTIAL

The next Spring Breakup Outlook is scheduled at 2PM AKDT Friday April 24th, 2020

Alaska Community Wind Tool

Yesterday UAF's International Arctic Research Center announced a new website tool for exploring the wind climate of recent decades across Alaska, as well as projected changes to wind behavior in the rest of this century.  It's a nice resource for learning about wind in many of Alaska's inhabited places, with data available from 67 sites.

http://windtool.accap.uaf.edu/

The aspect that interests me the most is the monthly wind rose visualization; this is a great way to see how wind typically varies through the year in different places.  For instance, the diagrams from Utqiaġvik, Deadhorse, and Kaktovik show interesting differences that I wasn't fully aware of until now.  Click to enlarge the images below.

At Utqiaġvik, easterly to northeasterly winds are strongly dominant at all times of the year, with the preferred direction being east-northeast from September through May, and east in June through August.  But at the two sites farther east along the Arctic coast, the wind rose is strongly bi-directional in most months: mostly ENE or WSW at Deadhorse, and approximately WNW or E at Kaktovik.  It's amazing how reliable the ENE direction is in Deadhorse in May through July; the same is true for the easterlies in Kaktovik in May and June.

Another spot with a remarkably reliable wind regime is Unalakleet in the winter; it's basically all easterlies all the time, and often quite strong.  In this case the wind regime is localized, being created by cold air flowing down the Kaltag Portage from the interior.

CRN Data Visualization

Here's a follow-up to a post from a few weeks ago, where I mentioned that I was working on a web interface to visualize the high-quality climate data coming out of the Alaska CRN network.  Here's a first draft:

http://worldagweather.com/crn/

Note that the page is not designed for mobile viewing - sorry if that's an inconvenience.  And various refinements are needed; this is just a first attempt that provides static, pre-generated images for fixed intervals.  I'd like to move to a more interactive charting capability at some point, but that will depend on how much time I have.

Also note that I haven't included the two newest sites in southwestern Alaska, one in the Yukon-Kuskokwim Delta, and one at Aleknagik (near Dillingham).  This is because I prefer to have at least a couple of years of data to calculate a seasonal "normal" for each of the weather variables.

The page should update every day by around 7am AKST, but let me know if you see any problems.

Winter Hangs On

Winter is hanging on with considerable tenacity across Alaska, with another strong trough bringing a blast of renewed cold air from west to east yesterday and today.  Nome hasn't climbed out of the single digits Fahrenheit yet today, and Fairbanks is hovering near 15°F in the westerly breeze this evening.  These are very chilly conditions for the time of year, although in Fairbanks it's nowhere near the level of April 2013, when the 7th of the month brought a high of only 11°F and a low of -10°F.

This morning's 500m analysis from Environment Canada shows the upper-level trough centered over the Brooks Range, and the plotted observations indicate a 500mb temperature of -42°C at both Fairbanks and Anchorage (click to enlarge below).  This is the coldest mid-tropospheric air so late in the winter since 2013 at both sites, and it's a continued symptom of the strong and cold Arctic vortex that has dominated the boreal high latitudes for most of the winter.

But on the bright side for those who are tired of all this, a huge warm-up is looking quite likely for next week.  Remarkably, NOAA's probabilistic forecast is showing a median temperature forecast of 62°F in Fairbanks next Friday (the 17th), and that means they're calculating a 50-50 chance that it will be even warmer than that.  We'll see... 10 days is a long way out in Alaska weather forecasts.

Update 4/7: here's the link for the probabilistic forecast:

https://sats.nws.noaa.gov/~naefs_ekdmos/meteoform_naefs_v2_oper.php

The probability distribution values can be accessed via "Text Products" on the left menu.  Here's yesterday's Max/Min Temp CDF (Cumulative Distribution Function) product for Fairbanks:

Cold and Snow

As is only fitting, winter is coming to an end with another spell of unusual cold in much of Alaska; here are a few notable reports from the past several days.

-44°F  Umiat RAWS
-36°F  Eagle COOP
-34°F  Chicken COOP
-34°F  Tok 70SE CRN
-32°F  Salcha RAWS
-23°F  Goldstream Creek COOP

In Fairbanks, March is ending as the fourth month in a row with average temperature below the 1981-2010 normal, and it's the coldest first quarter of the year since 2007.

March was also a very snowy month in Fairbanks, as Rick Thoman illustrated nicely in the following plot (via Rick's Twitter feed); click to enlarge.

With 27.5 inches of snow, March was the snowiest month of the winter in Fairbanks.  Since the winter of 1929-30, this has happened 8 times before, so it's unusual but not rare.  Average (median) March snowfall is less than 5 inches, but it's a very skewed distribution, with occasionally much more occurring.

April is even more skewed, of course; the median is only 1.7", and yet even April has been the snowiest month of the winter on 4 occasions - most recently in 2008 (14.7").

Here's a frequency histogram of the snowiest month of the winter in Fairbanks, Bettles, and Anchorage.  Fairbanks stands out in terms of having October as the snowiest month rather frequently; I find this particularly surprising as rain is more common than snow in the first week or so of October in Fairbanks.

Other interesting features of the chart include the notable November-December peak in Bettles, and the enhanced frequency in late winter (February through April) in Anchorage.  Surprisingly, March has more often been the snowiest month than January in Anchorage, and average March snowfall lags only very slightly behind January.  I suspect part of this is an artifact of sampling variability, but there may well be a physical reason why January tends to underperform for snow; I don't think it's as simple as "it's too cold to snow", but if readers have any ideas, I'm all ears.

Why Cold Now - Part 3

In the first two installments of this discussion (here and here), I made a few comments about Alaska's surprisingly cold winter and the difficulty of finding a good explanation for it.  Of course from one perspective the reasons are obvious: the circulation pattern produced more northerly and westerly flow (and less southerly flow) than usual over the state, leading to a much-reduced influence of mild Pacific air.

The pattern is illustrated by the 500mb height anomaly map, which shows an unusual trough centered near the southeastern interior and a strong ridge over the central North Pacific.

A very strong westerly flow is implied between the ridge and the trough, and this strong and stable jet stream pushed warm air eastward into Canada and the Lower 48 rather than taking occasional (or frequent) northward excursions into Alaska.  Here's a map of the vector wind anomaly at 250mb; the westerly flow was more than 12 m/s stronger than normal to the south of Alaska.

For reference, here's the normal upper-level wind pattern in winter:

The map below shows the resulting temperature anomaly pattern, which really highlights that the configuration was "perfect" for bringing cold to the heart of Alaska; the state experienced an "island of cold" in a sea of unusual warmth (and I'm not showing Eurasia, which was much warmer still).

Another key aspect of the flow regime was a positive phase of the Arctic Oscillation (AO), indicating that (i) low pressure was unusually low in the Arctic, (ii) the mid-latitude westerly jet stream was unusually strong, and (iii) there was a strong contrast between unusual warmth in the mid-latitudes and relatively cold conditions in the Arctic.  The positive AO anomaly became increasingly pronounced as winter progressed, and the daily AO index reached all-time record positive values in February.

The AO phase has a modest connection to Alaska temperatures in winter, as illustrated by the chart below.  Note that I'm using detrended temperatures based on data since 1950, and yes, it was the coldest winter relative to trend since 1950.  However, the trend since 1950 is very steep (over 1°F/decade), and this winter's cold was less remarkable when compared to a longer-term trend or simply a long-term average.

The chart indicates that below-trend temperatures tend to be favored when the AO phase is positive, and this provides a reasonable connection between Alaska's cold winter and the larger Hemispheric flow pattern.  However, it's interesting to see that this winter's flow pattern over Alaska was not particularly well-aligned with the typical positive AO pattern.  The map below shows the average 500mb height departure from normal in the top 10 positive AO winters of the past, and while the same strong north-south gradient is evident across the North Pacific, the AO pattern usually has a trough axis near the Chukchi and Bering Seas rather than over eastern Alaska.

The temperature pattern associated with the positive AO phase reveals a cold signal over southwestern Alaska (see below), which is certainly consistent with what happened this winter; but it seems the positive AO is not typically associated with more widespread cold across the interior and north.  As an aside, it's interesting to see a negative PDO-like temperature pattern across the North Pacific in these winters, and indeed the PDO was negative in 7 of the 10 winters (as it was this winter).

A closely related, and significant, feature of the Northern Hemisphere winter was the unusual strength of the polar vortex up in the stratosphere.  The stratospheric polar vortex forms every winter and has a fairly close connection to the lower-atmosphere AO phase, but the two don't always behave in tandem over the course of a winter.  However, this winter the polar vortex was strongly coupled, especially in late winter, and the upper-level vortex became remarkably intense by late winter - see below.

Does the presence of a strong stratospheric vortex alter or magnify the AO temperature signal over Alaska?  Not really; the map below shows winters with both a positive AO phase AND a significantly stronger than normal stratospheric vortex, and it's a very similar signal to the AO pattern by itself.

So are there any winters in the historical record with a similar strong polar vortex/AO phase and more widespread and pronounced cold in Alaska?  The answer is yes; the winter of 1989-1990 was notably cold in Alaska, and the AO became increasingly positive (indeed extremely so) as winter wore on, similar to this winter.  The previous winter, 1988-1989, is probably more memorable to Alaskans because of the incredible cold snap in January 1989, but the subsequent winter of 1989-90 was actually colder overall, and February 1990 was bitterly cold (easily the coldest February on record for the state).

Here are the 500mb and surface temperature maps for 1989-1990: the match is by no means perfect, but it's a good deal better over the North Pacific and Alaska domains than with the positive AO composites.  Notice in particular the relative absence of cold in southeastern Alaska and the Aleutians, and the warm region over the northeastern North Pacific to the west of Oregon - all similar to this winter.

The similarities between these two winters, separated by exactly 30 years, are sufficiently intriguing - especially in regard to the dramatic strengthening of the AO phase in late winter - that I'll have to dig deeper and try to understand what was going on.  And that means readers can be on the lookout for Part 4 of this discussion in due course.

CRN Data Visualization

Spare time for new blog posts has been in short supply lately, but I'll mention a little project I've been working on that may be of some interest.  As many readers know, NOAA's Climate Reference Network of top-quality climate observing instruments has been gradually adding sites in Alaska over the past several years, and the state's network is now up to 22 sites.  Here's a map (click to enlarge):

All of the data is readily available from NCEI, but options to visualize the data appear to be very limited.  I'm aiming, therefore, to put together a simple interface that provides some charting and perhaps mapping capabilities to summarize the wealth of climate monitoring data that's flowing from the CRN instruments.

Here are a few prototype charts showing departures from normal of climate variables observed at the CRN site near Fairbanks over the past several months.  The seasonal normals are calculated from the full period of record, which is already nearly 18 years long for temperature, precipitation, and solar radiation, but only 10 years for wind speed and humidity.  Click to enlarge the images.

Here's precipitation since the beginning of the year.

Look for more posts on the CRN data in (hopefully) the not-too-distant future.

Why Cold Now - Part 2

Back in late January I raised the question of why this winter produced persistent cold over Alaska; it's such a dramatic change and stark contrast to other recent winters that it begs for attempts to explain it.  After all, if we can't explain these things even in hindsight, what hope do we have (speaking personally) of predicting them?

To confirm the point that this winter was indeed remarkably cold in Alaska, the chart below shows the December-February statewide average temperature according to NOAA's climate division data.  In absolute terms, the winter was the coldest since 1998-99, and compared to the trailing 30-year average it was the coldest in 50 years (since 1969-70).  One could argue that this winter was about a once-in-a-generation cold winter relative to the modern warmer climate; and if background warming continues (as seems inevitable), it may be a very long time before it's this cold again.  Click to enlarge:

In my earlier post I discussed the apparent lack of connection of the winter pattern with tropical ocean temperature signals.  Tropical oceanic and atmospheric phenomena are often closely followed by long-range forecasters because of their slow and predictable progression, together with known (or supposed) mechanisms for influencing higher latitude weather.

In addition to sea surface temperature (SST) patterns, forecasters often track the behavior of tropical convection, i.e. regions of disturbed weather, including large clusters of thunderstorms.  One tool for such tracking is the so-called velocity potential of the upper troposphere; the velocity potential simply isolates the divergent part of the flow, which reveals where convection is relatively active (divergence aloft) or inactive (convergence aloft).

To illustrate, here's the departure from normal of this winter's velocity potential (VP) at 200mb.  Blue shading (negative VP) indicates unusual divergence aloft, implying more rain than normal (hence the locust outbreaks in East Africa), and yellow/orange indicates unusual convergence aloft (less rain than normal).  Note that VP is only useful in this sense in the tropics, because large-scale weather disturbances outside the tropics are not typically dominated by convective processes.

Now to the point of this discussion: it's worthwhile examining past cold winters in Alaska to see if there are similarities in the VP patterns.  If there are, then we might argue that the tropical convection was linked to the outcome in Alaska; and such a finding would not be controversial.

Here are the VP anomaly maps for Alaska's three coldest winters relative to trend since 1980; the lack of good satellite data prior to 1980 makes me reluctant to look at the VP analyses from earlier years.

Unfortunately there are no obvious similarities between the VP patterns in these years.  1989-90 perhaps comes closest to 2019-20, with enhanced rainfall in the Indian Ocean and suppressed convection over the Maritime Continent, but 1989-90 did not have enhanced activity over the central Pacific or over most of Africa.

Here's what a good match looks like: the winter of 2002-03 had similar tropical VP patterns to 2019-20, but it was one of the very warmest winters on record in Alaska.

To pursue a more objective approach, I calculated the similarity to 2019-20 of the VP anomalies along the equator, and none of the 3 cold years is in the top 10 matches among 40 years of data (although 1989-90 comes closest).  And just to put a nail in the coffin of this hypothesis, here is a map of winter patterns in those top 10 matches, i.e. the 10 winters with the most similar VP to 2019-20.

The pattern is about as opposite as it could be for northwestern North America; contrast the 2019-20 maps below.  So even if we had known the VP patterns perfectly in advance, an analog approach like this would have predicted another warm winter for Alaska.

In summary, this cursory analysis provides absolutely no evidence that Alaska's cold winter was directly linked to patterns of tropical rainfall activity, and this is a disappointment.  It's not entirely surprising, though, because the SST patterns provided no help either, and there are very close links between SST anomalies and tropical convective regimes.

In Part 3 of this analysis, I'll look at another aspect of the winter that became increasingly unusual as the winter progressed, and that's the strongly positive Arctic Oscillation and increasingly intense stratospheric polar vortex.  In this case there is definitely some demonstrable correlation to Alaska's winter weather, but I think we'll find that it still leaves many questions unresolved.

Winter Hangs On

Unusual cold is hanging on with remarkable tenacity in Alaska this winter, and last night saw temperatures worthy of the depths of winter in many locations.  Here are a few examples of notable low temperatures today:

-50°F  Bettles
-45°F  Tanana
-44°F  Chalkyitsik RAWS
-42°F  Nenana

And in and around Fairbanks:
-46°F  Salcha RAWS
-41°F  North Pole
-39°F  Goldstream Creek
-39°F  Smith Lake at UAF
-38°F  Fairbanks airport

Here's the temperature trace from the past two weeks at Smith Lake (UAF's North Campus).  The temperature did not break -10°F today, which is pretty impressive for the time of year.

Of course clear skies at this time of year produce large diurnal temperature ranges, as seen in the Smith Lake data.  The Chalkyitsik RAWS (about 20 miles east of Fort Yukon) often produces spectacular examples, and just the other day this site saw temperatures vary from a low of -49°F on two consecutive nights to a high of 0°F in the intervening day.  When looking at days with a high temperature of 0°F or below, this is the largest diurnal range on record for the site (data since 1997).

The chart below shows daily Chalkyitsik temperatures so far this winter, as compared to seasonal normals (obtained from the 1997-2018 history).

Two features are striking: the persistence of unusual cold since mid-December, and the expansion of the daily temperature range since early February.  The latter is entirely in keeping with climatology, which shows a pronounced rise in the average diurnal range in late winter; in fact it's remarkable to see the normal high temperature bottom out very soon after winter solstice, while the normal low temperature doesn't show any meaningful rise until well into February.